The present invention is directed to a photodetector and more particularly to a photodetector having spatially varying wavelength absorption properties within a single semiconductor structure. The present invention is further directed to a method of making such a photodetector.
High-speed networking is required both in local areas and at points of access to high-speed metropolitan-area networks. Active optical components are needed to detect optical signals on many wavelengths in a cost-effective manner. A very broad bandwidth needs to be addressed to take full advantage of emerging optical fiber technologies with low loss from 1300 to 1600 nm and beyond.
Traditional techniques for wavelength-division demultiplexing have used some sort of refracting or diffracting optical element to break up incoming signals by wavelength to apply the various wavelengths to multiple photodetectors. However, such techniques have resulted in devices which are expensive to manufacture.
U.S. patent application Ser. No. 09/833,078 to Thompson et al, filed Apr. 12, 2001, entitled xe2x80x9cA method for locally modifying the effective bandgap energy in indium gallium arsenide phosphide (InGaAsP) quantum well structures,xe2x80x9d and published Mar. 14, 2002, as U.S. Ser. No. 2002/0030185 A1, whose entire disclosure is hereby incorporated by reference into the present disclosure, teaches a method for locally modifying the effective bandgap energy of indium gallium arsenide phosphide (InGaAsP) quantum well structures. That method allows the integration of multiple optoelectronic devices within a single structure, each comprising a quantum well structure.
In one embodiment, as shown in FIG. 1A, an InGaAsP multiple quantum well structure 104 formed on a substrate 102 is overlaid by an InP (indium phosphide) defect layer 106 having point defects 108, which are donor-like phosphorus antisites or acceptor-like indium vacancies. Rapid thermal annealing (RTA) is carried out under a flowing nitrogen ambient, using a halogen lamp rapid thermal annealing system. During the rapid thermal annealing, the point defects 108 in the defect layer 106 diffuse into the active region of the quantum well structure 104 and modify its composite structure. The controlled inter-diffusion process causes a large increase in the bandgap energy of the quantum well active region, called a wavelength blue shift.
Another embodiment, as shown in FIG. 1B, uses two defect types, one to generate a wavelength blue shift and the other to decrease carrier lifetime. A first InP defect layer 110 contains slowly diffusing vacancy defects 114, while a second InP defect layer 112 includes rapidly diffusing group V interstitial defects 116. Rapid thermal annealing causes both types of defects to diffuse into the quantum well active region.
However, the above-noted problem of the demultiplexing photodetector has not yet been solved.
It will be readily apparent from the above that a need exists in the art for a compact, inexpensive demultiplexing photodetector. It is therefore an object of the present invention to provide a demultiplexing photodetector in a single semiconductor structure. It is another object of the invention to provide an inexpensive manufacturing technique for such a photodetector.
To achieve the above and other objects, the present invention is directed to a photodetector in which multiple wavelength detecting regions are integrated into a monolithic semiconductor structure, as well as to a method of making such a photodetector using the technique described above or any other suitable intermixing technique. The photodetector uses the above-described techniques to provide multiple photodetecting regions and thus to provide an integrated serial single-waveguide ultra-broadband photodetector. The invention exploits the above-described technique to shift the absorption edge locally. A waveguide is used to carry the light along the length of the device. All materials can be grown semi-insulating and on a semi-insulating substrate to achieve electrical isolation between different wavelength-detecting regions. The length of the device and thus the bandgap grading length achieved by intermixing are chosen so that all high-energy photons are absorbed in the early absorption regions. Wavelength-resolving capability then given approximately by:
grading rate (xcexcm per xcexcm)/(loss in microns).
For 2000 cmxe2x88x921, hence 0.2 xcexcmxe2x88x921, a spatial grading rate of 10xe2x88x924 xcexcm/xcexcm would give a resolution of about 5xc3x9710xe2x88x924 xcexcm, or about 0.5 nm. This device could demultiplex and detect the entire spectrum 1.3-1.6 xcexcm (hence 0.3 xcexcm of bandwidth) in a device length of 600 xcexcm, hence less than 1 mm. Achieving this resolution will depend on the sharpness of the local spectra of the photoconductive materials and on the sophistication of electronic signal-processing capabilities. Nonlinearity in grading or other properties may be precisely compensated using reciprocally nonlinear variable contact spacing.